1. Field of the Invention
The present invention relates to an on-vehicle navigation system having a route searching function.
2. Description of Background Information
There is a well known on-vehicle navigation system which is constructed such that map data including road data produced by converting each point on roads of a map to numerical values is stored into a memory medium such as a CD-ROM or the like, a group of map data in an area of a predetermined range including the present location of a vehicle is read out from the memory medium and is displayed on a display as a map of a region around the present location of the vehicle while recognizing the present location of the vehicle, and a vehicle's location indicative of the present location of the vehicle is automatically displayed on the map.
Since the conventional on-vehicle navigation system constitutes what is called a locating system which merely shows the present location of the vehicle to passengers by means of a display, an on-vehicle navigation system having a function for automatically searching for traveling routes to a destination and for performing guidance has been developed recently. When the route is searched for, a calculating method such as a Dijkstra method or the like for obtaining the shortest distance is generally used. According to this method, weights are produced for a road between crossings in consideration of road attributions such as express highway, distance, road width, and the like and a route in which the sum of the weights is the minimum value is derived as a guiding route.
A conventional route searching function will now be specifically explained. It is now assumed that there are a plurality of roads as shown by solid lines in FIG. 1 and intersection points A, B, . . . , and E are set to points as crossings of the roads. One road between the crossings is now called a "road unit". A numerical value written to each road unit denotes a weight of the road unit. For running from a point "A" (selected as a start point) to a point "B", the following three routes 1, 2, and 3 are conceivable.
Route 1: A.fwdarw.C.fwdarw.B PA1 Route 2: A.fwdarw.D.fwdarw.B PA1 Route 3: A.fwdarw.C.fwdarw.E.fwdarw.B PA1 Route 1: 3+5=8 PA1 Route 2: 8+5=13 PA1 Route 3: 3+3+5=11
The sum of weights for each route is shown as follows:
Among the three routes, since the sum of the weights of the route 1 which is shown by bold lines in FIG. 2 is the minimum, the route 1 is to be selected as a guiding route.
In such a conventional route searching operation, however, there is a problem such that even in such a case that the user, e.g., a driver or the like, can reach the destination in a shorter time by taking a route selected according to the user's run experiences than following the guiding route, the same route is always automatically selected as a guiding route from the same point to the same destination as long as a new road attribute which changes the weight is not generated.